A light-sensitive sheet comprising a support and a light-sensitive layer is widely used as a material for forming an image pattern. In more detail, the light-sensitive sheets are used to prepare printed circuit boards, display elements (e.g., color filter, column, rib, spacer, partition) or other imaging products (e.g., printing plate, hologram, micromachine, proof).
A printed circuit board is prepared by forming a circuit pattern with a photo-litho process. The photo-litho process uses the light-sensitive sheet comprising a transparent support (e.g., polyethylene terephthalate film) and a light-sensitive layer. The light-sensitive (transferring) sheet is sometimes referred to as a dry film-resist in the field of photo-litho process.
A printed circuit board having through-holes can be prepared, for example by the steps of:
(1) making a through-hole on a substrate (e.g., copper-clad laminate) of printed circuit board;
(2) forming a metal plating layer on an inner wall of the through-holes;
(3) laminating the light-sensitive layer of a light-sensitive sheet on a surface of the substrate;
(4) exposing the light-sensitive laminate to light corresponding to a pattern comprising an area where a circuit pattern is formed and an area of the through-holes, to harden the light-sensitive layer within the areas;
(5) peeling the transparent support from the light-sensitive sheet;
(6) dissolving the unhardened area in a developing solution to remove the unhardened area of the light-sensitive layer while keeping the hardened areas where the circuit pattern is formed and where the through-hole is located (the hardened layer remaining in the area where the through-hole is located is often referred to as “tenting film”);
(7) subjecting the bared metal plating layer of the substrate to an etching treatment; and
(8) removing the light-sensitive layer within the hardened areas to form the circuit pattern on the substrate.
A metal plating layer in a through-hole can be protected according to a method using a tenting film formed at step (6). The method is generally referred to as a tenting method.
A multi-layered printed circuit board often has a via-hole in place of (or in addition to) the through-hole. The via-hole connects the layers in the circuit. A metal plating layer in the via-hole should be also protected with the tenting film to form a circuit pattern.
A recent printed circuit board is required to be more densely integrated. Accordingly, a light-sensitive sheet is required to give a circuit pattern with high resolution. A thin light-sensitive layer is effective in improving resolution of the light-sensitive sheet. On the other hand, a tenting film formed by hardening the light-sensitive layer has a function of protecting the through-hole or the via-hole in the printed circuit board, as is described above. A thin tenting film formed from the thin light-sensitive layer is liable to break while dissolving the unhardened area of the light-sensitive layer in a developing solution to remove the unhardened area, or while subjecting the bared metal plating layer to an etching treatment.
Therefore, the light-sensitive sheet has been improved to form a strong tenting film as well as a pattern with high resolution.
Japanese Patent Provisional Publication No. 10(1998)-142789 discloses a light-sensitive sheet improved in components of the light-sensitive layer. The light-sensitive layer comprises a binder having carboxyl group, monomers comprising a specific vinylurethane compound and a specific acrylate compound, and a photo-polymerization initiator.
Japanese Patent Provisional Publication No. 8(1996)-54732 discloses a light-sensitive sheet improved in a layered structure. The light-sensitive sheet comprises a support, a first light-sensitive layer and a second light-sensitive layer in this order. The first light-sensitive layer is soluble in an alkaline solution, has low fluidity when heated, and is sensitive to an active energy ray. The second light-sensitive layer is soluble in an alkaline solution, and has high fluidity when heated, and is sensitive to an active energy ray. The light-sensitive sheet has through-holes. The through-holes are filled with the second light-sensitive layer to protect the metal layer in the holes.
Japanese Patent Provisional Publication No. 10(1998)-111573 discloses a light-sensitive resin laminate comprising a support, a first light-sensitive layer and a second light-sensitive layer. The two light-sensitive layers contain different vinyl copolymers, but they further contain the same specific monofunctional monomer. The light-sensitive laminate is improved in adhesion and resolution.
Japanese Patent Provisional Publication No. 3(1991)-17650 discloses a photo-resist comprising a support, a non-adhesive light-sensitive layer and an adhesive light-sensitive layer. The publication also discloses a process of preparation of a printed circuit board from the photo-resist. According to the publication, the photo-resist is improved in tenting performance and the resolution.
In the field of printing plates, Japanese Patent Publication No. 37(1962)-1306 discloses a light-sensitive plate for preparation of a printing relief. The light-sensitive plate comprises a substrate and at least two photo-polymerizable layers (thickness: 25 μm to 2.5 mm). The photo-polymerizable layers are polymerized at different rates. The photo-polymerizable layer near the substrate can be polymerized more rapidly than the other layer. The publication reports that the light-sensitive plate gives a relief image in which an area of the top face (printing face) of the relief is smaller than an area of the bottom (base) of the relief.